A well-known piece of machine shop equipment is the Bridgeport Series One vertical milling machine which has been in the marketplace since the 1930's. The tool spindle of the Series One milling machine, to which an end mill is typically mounted for cutting tool applications, is driven by a motor which is interfaced to a pair of pulleys having stepped configurations wherein each pulley defines four (4) annular surfaces of different diameter. Particularly, the Series One comprises a pulley housing which includes a drive motor connected thereto. A drive shaft extends downwardly from the drive motor into the pulley housing and includes a first stepped pulley attached to the distal end thereof which is disposed within the pulley housing. The tool spindle, which extends downwardly from the pulley housing, includes a second stepped pulley attached to the end thereof disposed within the housing. As such, the second stepped pulley is also disposed within the pulley housing. The first and second stepped pulleys are interfaced to one another via a V-belt which extends between corresponding, i.e. horizontally aligned, pairs of annular surfaces of each of the pulleys. Since each of the stepped pulleys defines four (4) differently dimensioned annular surfaces, the Series One is capable of achieving four (4) distinct rotational speeds, with each speed being dependent upon the vertical position of the V-belt. Thus, when it is desired to adjust the rotationable speed of the tool spindle, it is necessary to manually reposition the V-belt upon the first and second stepped pulleys. In many instances, the discreet speeds obtainable with the Series One are inappropriate for desired end mill cutting applications. Additionally, the requirement to manually reposition the V-belt upon the stepped pulleys is time consuming.
Though many modern milling machines are provided with variable speed drives, large numbers of the Bridgeport Series One milling machine are still in operation and do not possess the variable speed capacity of the modern milling machines. The present invention addresses the deficiencies associated with Bridgeport Series One vertical milling machines relating to the adjustment of the rotational speed of the tool spindle.